Lecture 10 Anesthesia in Horses Flashcards
What are things you are interested when preping for horse anesthesia
- Presenting complaint
- Patient status
- Procedure
- IV catheter – jugular
- Wrap lower limbs
- Wash mouth
What premedications do you use for horses
- Alpha-2 agonist (one of the following):
- Xylazine
- Detomidine
- Romifidine
- Opoid
- Butorphanol
- Why would you use an alpha-2 agonist
- What are the onset of actions if given IV?
- Reasons:
- sedation
- anxiolysis
- adjunct analgesia
- muscle relaxation
- Onset of action 3-5 minutes (IV)
What is the onset of action and duration of action of:
- Xylazine
- Detomidine
- Romifidine
- Xylazine –
- Onset of action ~3-5 minutes,
- DOA ~30-60 minutes
- Detomidine –
- Onset of action ~5 minutes,
- DOA ~60 minutes
- Romifidine –
- Onset of action ~15 minutes,
- DOA ~2 hours
What are the cardiovascular effects of alphla-2 agonists
- Initial hypertension then hypotension
- Bradycardia,
- 2nd degree atrio-ventricular block
- P waves with no QRS
- ↓ cardiac output
- What type of opioid is butorphanol
- Why use this opoid and not others
- What is another function of butorphanol other than analgesia?
- Mu antagonist/kappa agonist
- Other opoids are less mac sparing in horses and causes excitement
- Makes horse ‘plants’ feet
- (They don’t move their and place them down)
What do you use for induction on horses
- Ketamine
- benzodiazepine
- midazolam or (this one used most commonly now)
- diazepam
- Guafenesin (GG)
- What is guafenesin?
- What side effects does it have?
- How is it administered?
- Why administered that way?
- Central muscle relaxant
- Mild CV/respiratory effects
- 5% solution in 1 liter 5% dextrose
- 10% or > causes hemolysis & thrombophlebitis
How are the induction medications given?
- Bolus guafenesin 25-50mg/kg (250-500ml/horse)
- Then bolus Ketamine + benzodiazepine (midazolam or diazepam) (KM or KD)
How are horses intubated?
Blind intubation
- Extend head/neck in lateral recumbency
- Mouth speculum
- Tongue thru interdental space
- Over tongue, past cheek teeth, thru glottis
- Resistance?
- Pull back slightly, rotate 90° & advance
- DO NOT FORCE
- Inflate ET cuff
- Ventilatory support through Demand valve – IPPV with O2
- They become hypoxic shortly after intubation
How are horses positioned
- use hoist
- position on table
- Want to have even distribution on muscles
- want to maintain blood flow
- Dont want pressure on nerves
What are different ways to administer Intra-op Analgesia
- Regional anesthesia
- Can be limited by needing use of limbs for recovery
- Constant Rate infusions
- Lidocaine
- Ketamine
- Dexmedetomidine
What Intra-op Monitoring is done in horses
- ECG
- SpO2
- EtCO2
- Inhalant agent monitoring –
- Inspired/Expired Isoflurane
- Invasive (direct) BP monitoring always
What must you remember when monitoring EtCO2 in horses
EtCO2 is 10-15 less than PaCO2
What is the minimum MAP you want to maintain in horses
70mmHg (60 in small animals)
How do you treat hypotension in horses
- IV fluids
- Multi-modal analgesia/anesthesia
- To decrease amount of isoflurane used because it decreases BP
- Dobutamine CRI
How does dobutamine work to treat hypotension?
- β1 > β2 - ↑ ↑ contractility & cardiac output, BP
- Horses have a lot of sererve so can do this
- Do not want peripheral vasoconstriciton so wont use norepinephrine and epinerphine to
Why do horses get a lot of Respiratory Insufficiency during anesthesia
- Anatomy, size & weight => alterations in PaCO2 & PaO2
- Compression atelectasis
- Dorsal > lateral recumbency
- Anesthetics depress
- respiratory drive
- respiratory muscle function
- ventilation (rate, volume)
- response to hypercarbia/hypoxia
What are 4 causes of hypoxemia in horses
- hypoventilation
- ventilation-perfusion (VA/Q)mismatching**
- right-to-left shunting of blood flow (Q)
- diffusion impairment
Explain what happens if you have:
- V/Q = 1
- V/Q > 1
- V/Q < 1
- V/Q = 1 =>
- ventitlation & perfusion well matched
- V/Q > 1 =>
- lung ventilated but not well perfused (dead space ventilation)
- V/Q < 1 =>
- lung well perfused but not ventilated
Why is EtCO2 < PaCO2 in horses
30-40% TV goes to large airways not Involved in gas exchange
What happens with large colon torsions
- Colon twists
- Inflates with air
- Will push up on diaphragm
- Decreases TV and can increase PCO2
What will the V/Q ratio be for this
- V/Q < 1
- Lung perfused but not ventilated
When is Intermittent Positive Pressure Ventilation used in horses?
Always!
Why use Intermittent Positive Pressure Ventilation in horses?
- Hypoventilation & ↑PaCO2
- Consistent EtISO for stable anesthetic plane
- Maintain adequate PaO2
- O2 supplementation + IPPV required
What are the 4 aspects of recovering of anesthesia in horses
- Prevention from injury
- Positioning
- Airway & oxygen support
- Sedation/analgesia
How are horses prevented from injury
- padded recovery stall
- head/tail ropes
- head helmet
- mats
How are horses positioned for recovery
- Pull lower forelimb forward
- Takes pressure off triceps muscle
What do you consider when giving analgesia after surgery
- Type, length
- Degree of pain
- Limitations in choices/method of analgesia
- NSAIDS
- Opioids
- Limited by cost, side effects
What kind of sedation can you do for recovery of horses
- Acepromazine or
- Alpha-2 agonists
- Why use acepromazine for recovery sedation
- Negatives for using it?
- Positives:
- Milder sedative compared to alpha-2
- Less ataxia
- Hypotension?
- Negatives:
- No analgesia
- Slow onset of action
- IM or IV prior to disconnect from inhalant (need to plan to give before)
- Why use Alpha-2 agonists for recovery sedation
- Negatives?
- Positives:
- More profound sedation
- Analgesia
- Faster onset
- Negatives:
- More ataxia
- Oxygenation??
Why use IPPV during recovery
- Demand valve
- Allows IPPV with O2
- Rate of oxygen delivery
- Up to 200L/min
- Provides IPPV until return of spontaneous ventilation (ROSV)
- Low inspiratory flows
- Resistance to expiration
What rate do you put horses at during recovery to provide Oxygen insufflation
5-15 L/kg/min
What are causes of Airway Upper airway obstruction during recovery
- Displacement of the soft palate
- Nasal congestion
- Largyngeal hemiplegia
- Upper airway surgery
- High incidence in draft breeds
Why do horses get Displacement of the soft palate
- Obligate nose breathers
- Extubate when swallowing
- Allows replacement of SP
- UA noise ‘snoring’
What happens to the nasal mucosa during anesthesia in horses?
Nasal mucosa thickens from congestion/edema during anesthesia
What can you do to help alleviate upper airway obstruction due to nasal congestion
- Phenylephrine (neosynephrine) to decrease congestion
- Provide ‘stent’ or alternate airway
- Oral tracheal tube
- Naso-tracheal tube
- Naso-pharyngeal tube
Why use Phenylephrine (neosynephrine) for recovery in horses
- to decrease congestion
- Significantly decreases the need for nasopharyngeal intubation
By providing a ‘stent’ or alternate airway in horses during recovery, what are some reasons why you wouldnt or would have to be careful.
- •Risk of damage to ET tube (expensive)
- •Risk of damage to laryngeal structures
- •Risk of obstruction due to kinking
- •Affect ability to stand?
What horses would you use stents or alternate airways
- Large amounts of GI reflux (colic)
- UA hemorrgage
- +/- draft horses
What are the downfalls of Place naso-tracheal tube for recovery
- Risk of damage to laryngeal structures
- Risk of obstruction due to kinking
- Affect ability to stand?
What are the downfalls of placing a naso-pharyngeal tube for recovery
- Risk of damage/hemorrhage of ethmoids
- Due to incorrect placement
- Middle meatus vs ventral meatus
What are Recovery Complications
- Upper airway obstruction leading to pulmonary edema
- Post-anesthetic neuropathy
- poor positioning
- Post-anesthetic myopathy
- poor positioning
- Intra-operative hypotension
- Fracture
